Devices for electrical switching, such as circuit breakers, may need to survive a fault or short-circuit conditions, in which the electrical current through the device may be many times larger than the device's continuous current rating (the so-called rated current or handle rating). If such a fault current lasts even a few seconds, the conductive parts (e.g., electrical contacts) of the electrical device may be degraded or even melt to some extent. Debris and arcing may also possibly damage other device components that are in the vicinity of the electrical contacts.
One prior art remedy used to protect components has been to wrap a part of the contact arm 401 and to separately wrap the spring hook 403SH of the contact spring 403 of the electrical device with a fiberglass cloth 401C as shown in FIG. 4A. However, this is an expensive and labor intensive process, wherein an operator manually precuts and wraps the fiberglass cloth and wraps it onto the contact arm 401 and spring hook 403SH.
Furthermore, the fiberglass cloth 401C may become unraveled in some instances possibly causing other potential interferences (e.g., lodging between the contacts). Moreover, the fiberglass cloth 401C may not provide suitable protection in some respects. For example, the fiberglass cloth wrapping position on the contact arm 401 and the spring hook 403SH may not be consistent (e.g., may be operator dependent), and may leave one or more gaps exposing the contact arm 401 and/or spring tab 401ST to the electrical arc and debris. Moreover, the spring hook 403SH may be exposed. Further, the fiberglass cloth 401C may provide a relatively low degree of protection, as it may rapidly vaporize when contacted by an arc.
Another remedy utilized in the prior art is to include a stainless steel shielding apparatus 401SS as shown in FIG. 4B. However, the stainless steel shielding apparatus 401SS is electrically conductive. Therefore, the electrical arc can cause premature damage to it reducing the protection to the contact arm 401 and/or the spring hook 403SH. Moreover, because of difficulties in forming stainless steel, there may be relatively large gaps left open thus exposing the contact arm 401 and/or the spring hook 403SH to the electrical arc. Further, the stainless steel shielding apparatus 401SS involves a secondary operation to secure it in place (e.g., riveting) wherein a rivet 401R connects the stainless steel shielding apparatus 401SS to the contact arm 401. This riveting may weaken the contact arm 401 as the riveting operation may involve a hole (not shown) to be formed therein.
Thus, there is a need for improved shielding apparatus and shielded contact arm assemblies configured for use in electrical switching devices, such as circuit breakers.